Yeah, for an M4 Max I'd expect 1.5-2x (the CPU can only use about half the total bandwidth, roughly, so the GPU should still win if you discount initialization), though if memory serves the Metal API needed some special flag set in addition to the basic alignment requirement in order to actually directly use the same buffer without extra staging, at least when I last used it.
More generally, if your language builds a unified memory abstraction on top of systems that have physically heterogenous memory, make sure the cost model doesn't just include the nominal bandwidth differences (or asymptotic ones, if you do live profiling), but also the overhead of any extra staging buffers you use (explicitly or not, drivers and runtimes are not generally shy of allocating behind your back).
Yeah, for an M4 Max I'd expect 1.5-2x (the CPU can only use about half the total bandwidth, roughly, so the GPU should still win if you discount initialization), though if memory serves the Metal API needed some special flag set in addition to the basic alignment requirement in order to actually directly use the same buffer without extra staging, at least when I last used it.
More generally, if your language builds a unified memory abstraction on top of systems that have physically heterogenous memory, make sure the cost model doesn't just include the nominal bandwidth differences (or asymptotic ones, if you do live profiling), but also the overhead of any extra staging buffers you use (explicitly or not, drivers and runtimes are not generally shy of allocating behind your back).